Novel method of electrocoating hollow bodies

ABSTRACT

A METHOD OF ELECTROCOATING THE INTERIOR OF HOLLOW OBJECTS REQUIRING A COATING OF A WATER-SOLUBLE OR WATER-DISPERSIBLE COATING AGENT WHICH OBJECTS HAVE AT LEAST AN ELECTRICALLY CONDUCTIVE SURFACE WHEREIN THE IMPROVEMENT RESIDES IN THE USE OF AT LEAST ONE BIPOLAR AUXILIARY ELECTRODE WHICH EXTENDS INTO THE INTERIOR OF THE HOLLOW OBJECT.

June 15, 1971 F. WEHRMANN ETAL NOVEL METHOD OF ELECTROCOATING HOLLOWBODIES Filed Sept. 5, 1968 2 Sheets-Sheet 1 F I G 5 :3 F E G .4

la 4 5o 50 i; 1 I o o 80 INVENTORS FELIX WEHRMANN FRANZ AIGNER ELWJA.Azw/ ATTORNEYS Jllflfi F WEHR EIAL OVEL METHOD OF ELEGTROCOATING HOLLOWBODIES Filed Sept. 5, 1968 2 Sheets-sheet 2,

INVENTORS FELIX WEHRMANN FRANZ AIGNER ATTORNEYS United States Patent US.Cl. 204-181 12 Claims ABSTRACT OF THE DISCLOSURE A method ofelectrocoating the interior of hollow objects requiring a coating of awater-soluble or water-dispersible coating agent which objects have atleast an electrically conductive surface wherein the improvement residesin the use of at least one bipolar auxiliary electrode which extendsinto the interior of the hollow object.

PRIOR ART Known electrocoating methods face great difficulties whenattempting to provide the interior of hollow bodies with a coating. Forthe following discussions, it will be assumed that the coating agent isto be deposited on the anode which in this case will be the object to becoated and the counter-electrode is the electrode. The difiiculty incoating the interior of the hollow bodies resides in the fact that aminimum current density is required to form the coating. Since theelectrolytic resistance between the cathode and the outer surface of theobject to be coated is substantially lower than between the cathode andthe inner surface of the object to be coated, the current densitiesappearing on the surfaces also vary considerably. The entire potentialdrop is practically between the cathode and the outer surface of theobject to be coated. Only with increasing outside coating is thepotential drop displaced partly into the interior of the object to becoated, so that only a partial and irregular coating of the innersurfaces is achieved. One attempt by the prior art to solve this problemrequired the use of an auxiliary cathode in the interior of the hollowbody to be coated. However, this solution is expensive and hasconsiderable technical disadvantages.

OBJECTS OF THE INVENTION It is an object of the invention to provide aneconomical and simple method of coating the interior of hollow bodies.

It is another object of the invention to provide a novel electrocoatingapparatus wherein the interior of hollow bodies are coated with the aidof a bipolar electrode.

These and other objects and advantages of the invention will becomeobvious from the following detailed description.

THE INVENTION The method of the invention for electrocoating conductivesurfaces of a hollow body with a coating material comprises immersing ahollow object having at least an electrically conductive surface whichacts as an electrode in an electrically conductive coating bathcontaining an organic coating agent to be deposited on the hollow objectand provided with a counter-electrode, passing an electric currentthorugh the said coating bath and removing the coated hollow object fromthe coating bath, the coating of the interior of the hollow object beingeffected by at least one bipolar auxiliary electrode with one poleinside the hollow object and one pole outside the hollow 3,585,120Patented June 15, 1971 'ice object. Bipolar electrodes are electrodeswhich act on one end or side as anodes and on the other end or side ascathodes and such electrodes are known such as in electrometallurgy forrefining copper (Milazzo, Textbook of Electrochemistry) The method ofthe invention has great technical advantages compared to theconventional methods for coating hollow bodies. Since the bipolarauxiliary electrodes are not in metallic conductive contact with thecounterelectrodes of the hollow body to be located, there is no dangerof a short-circuit in case the bipolar auxiliary electrode comes incontact with the hollow body to be coated. Destruction of the hollowbody, damage to the rectifier, impulse generator or any other currentsource are thus impossible. Since the bipolar auxiliary electrodes aresmaller than conventional auxiliary electrodes connected in the directcircuit and are made of a less expensive material than the latter,considerable savings are possible. Moreover, the complicated assembly ofthe auxiliary electrodes immediately before the coating is eliminated.These bipolar auxiliary electrodes can rather be mounted at the mostfavorable time of the manufacturing process of the hollow body to becoated. For example, it is possible, to mount the auxiliary electrodesin the wall of a car body before the wall is assembled in the car body.With the method according to the invention it is also possible to coatcavities, for example, of car bodies, which could only beunsatisfactorily coated heretofore or not at all.

The following materials may be used for the production of water-solubleor water-dispersible coating agents in the electrically conductive bathsfor the method of the invention: organic and inorganic pigments, cuttingagents, such as rutile, anatase, lithopone, blanc fix, kaolin, heavyspar, zinc sulfide, carbon black, iron oxide dye, zinc chromate, leadcyanamide, heliogen blue, as well as soluble dyes, such as varnish dye,in a ratio of pigment or dye to solid resin of 0 to 5.0:1.

The following materials may also be organic film forming agents:water-soluble or water-dispersible synthetic resins and natural resinsas well as combinations thereof, water-soluble or water-dispersiblevegetable or animal oils and fats and their transformation products,such as linseed oil and sardine oil, dehydrogenated castor oil,watersoluble or water-dispersible natural resins and theirtransformation products, such as shellac, resins, soaps and resinesters; water-soluble or water-dispersible synthetic resins modifiedwith natural resins, water-soluble or waterdispersible maleinate resin;water-soluble or water-dispersible saturated and unsaturated polyesters;water-soluble or water-dispersible oil-free and styrenized alkyd resins,water soluble or water dispersible alkyd resins with synthetic fattyacids; water-soluble or water-dispersible alkyd resins with vegetableand animal fatty acids; watersoluble or water-dispersible acrylic alkydresins; watersoluble or water dispersible silicone akyd resins;watersoluble or water dispersible phenol resins and novolaks;water-soluble or water-dispersible butanolized resol resins modifiedwith fatty acids or by incorporation of polyesters; water-soluble orwater-dispersible resols and coldhardening phenol resins; water-solubleor Water dispersible alkyl pheno resins; water-soluble orwater-dispersible terpene-phenol resins; water-soluble orwater-dispersible plasticized or unplasticized urea resins;water-soluble or water-dispersible melamine resins; water-soluble orwater dispersible polyvinyl acetals; water-soluble or water-dispersiblepolyvinyl chlorides and polyvinyl chloride mixed polymerizates;water-soluble or water-dispersible polyvinyl acetates and otherpolyvinyl esters; water-soluble or water- 'dispersible polyvinyl ethers;Water-soluble or waterdispersible polyvinyl carbazoles; water-soluble orwaterdispersible polyacrylic resins and resins of polyacrylic acidderivatives, such as water-soluble or water-dispersible polymethacrylicacids; water-soluble or water dispersible polystyrenes; water-soluble orwater-dispersible polyethylcnes; water-soluble or water-dispersiblepolyisobutylenes; watersoluble or water-dispersible cumerone and indeneresins; water-soluble or water-dispersible ketone and aldehyde resins;water-soluble or water dispersible aromatic derivatives of formaldehyderesins; water-soluble or waterdispersible anilin resin; water-soluble orwater-dispersible carbamic acid resins; water-soluble orwater-dispersible sulfonamide resins; water-soluble or water-dispersiblechlorodiphenyl resins; water-soluble or water-dispersible polyamideresins, water-soluble or Water-dispersible polyaddition resins such aspolyurethane; water-soluble or water-dispersible epoxide resins andtheir transformation products; water-soluble or water-dispersiblenitrocellulose resins as well as other water-soluble orwater-dispersible cellulose resins; water-soluble or water-dispersiblenatural and synthetic rubber resins and their dirivatives such asbutadiene derivatives; water-soluble or water-dispersible siliconeresins; water-soluble or water-dispersible resins of halogenatedpolyethylene; and water-soluble or waterdispersible polycarbonates.

Instead of all of the above indicated natural or synthetic resins, theirwater-soluble or water-dispersible soaps or other derivatives may alsobe used as binders.

The electric current used in the method of the invention may be directcurrent and/or alternating current and/or pulsating current of any form.The use of alternating current and pulsating currents is fully describedin my copending US. patent applications Ser. No. 628,390 filed Apr. 4,1967 and Ser. No. 635,977 filed May 2, 1967, the disclosures of whichare hereby incorporated by reference thereto.

The bipolar electrodes may take any form. One embodiment is a copperwire with an iron mesh screen at one end as illustrated in FIG. 5. Aparticularly useful bipolar anode is a plastic pipe filled with steelwool and provided with openings along its length. The length of theflexible plastic pipe may be easily adjusted to conform to the form andsize of the cavity to be coated. One part of the plastic pipe willprotrude into the hollow body and the other end will protrude outsidethe cavity and the steel wool in the outer part will be coated duringthe coating process. In a subsequent coating process, the same electrodecan be arranged so that the coated portion is in the cavity to be coatedand uncoated portion is outside the cavity. During this coating process,the uncoated portion becomes coated and the coated portion is freed fromthe coating agent. This has the exceptional advantage of being able touse the bipolar electrode almost an infinite number of times.

The thickness of the coating on the interior of the hollow body can beregulated by varying the ratio of the surface area of the interior ofthe hollow body to the surface area of the coated portion of the bipolarauxiliary electrode.

Referring now to the drawings-- FIG. 1 illustrates a known apparatus forelectrocoating with direct current and FIG. 2 is an equivalent circuitdiagram of the apparatus of FIG. 1.

FIG. 3 illustrates a known apparatus for electrocoating interiors ofhollow bodies with direct current and FIG. 4 is the equivalent currentdiagram of the apparatus of FIG. 3.

FIG. 5 illustrates one embodiment of the apparatus of the invention tocoat the interior and exterior of a hollow object using direct currentand FIG. 6 is the equivalent current diagram of the apparatus of FIG. 5.

FIG. 7 is another embodiment of the apparatus of the invention forcoating hollow objects with direct current.

FIG. 8 is an embodiment of the apparatus of the invention for coating ahollow body with alternating current. FIG. 9 illustrates an embodimentof the invention in which two car bodies are coated simultaneously usinga pulsating current.

FIG. 10 illustrates an embodiment of the invention in which a car bodyis coated using direct current.

In the prior art embodiment of FIGS. 1 and 2, the coating vessel 1 actsas a cathode with respect to the electrically conductive object 2 to becoated. The current supply for object 2 is derived fromcurrent-collecting rail 3. The object 2 is a can which has only anopening 4 in the bottom and 5 in the top surface. After this can 2 hasbeen introduced into the coating bath, the current is turned on and thecoating of the surface beings. FIG. 2 shows the equivalent circuitdiagram of this arrangement wherein 1a denotes the cathode, 2a thecontact point of the coating bath with the outside of the object to becoates, 3a the connecting point of the current source with the object tobe coated, 4a the resistance of the coating bath outside the object tobe coated, 5a the resistance of the film deposited on the outer surfaceof the object, 6a the resistance of the liquid column in the opening ofthe object (corresponding to opening 4 or 5 in FIG. 1); 7a theresistance of the coating bath inside the object, 8a the resistance ofthe film deposited on the inner surface of the object. It can be readilyseen that with a very low external resistance almost all the entirecurrent flows between the cathode and the outer surface of the object,since the points 2a and 3a have practically the same potential.

The resistance of the liquid column 6a is substantially higher due tothe small cross section of the openings of the object than theresistance of the coating bath outside 4a and inside 7a of the object,and thus determines the current flowing on the inside. With increasingoutside coating, the resistance 5a rises and with it, also according toKirchhoff, the current flowing over the internal resistance, which isequal to the sum of the resistances 6a, 7a and 8a. Between the points 2aand 3a is thus formed a constantly increasing potential difference. Thehigher the film resistance 5a, the better is the coating on the inside.Since the film resistance cannot be increased indefinitely, however, thecurrent density inside the hollow body to be coated is limited to suchlow values that no sufiicient inside coating is possible.

FIGS. 3 and 4 illustrate the prior are method of overcoming thedeficiencies of the method of FIG. 1 by introducing an auxiliary cathodeinto the interior of the hollow body to be coaaed. In FIG. 3, the hollowbody 2 to be coated is connected by the current collecting rail 3 to thepositive pole of the current source and the bath vessel 1 and auxiliaryelectrode 6 are connected the negative pole of the current source. FIG.4 illustrates the equivalent circuit diagram for FIG. 3. FIG. 4 showsthe equivalent circuit diagram of this arrangement, where 1a denotes thecathode, 9a the auxiliary cathode, 3a the connecting point of thecurrent source with the object to be coated, 4a the resistance of thecoating bath outside the object to be coated, 5a the resistance of thefilm deposited on the outside of the object, 7a the resistance of thecoating bath inside the object and 8a the resistance of the filmdeposited on the inner surface of the object. The inside coating iselfected here independent of the outside coating. Points 1a and 9a havepermanently the same cathode potential.

This apparently very elegant solution has not only considerabletechnical disadvantages, but is also rather expensive. Since the fulloperating voltage is constantly applied to these auxiliary electrodes,they must be so designed that short-circuits are positively avoided. Inthe case of a short-circuit, not only is the rectifier jeopardized butthe auxiliary electrode and partly also the object to be coated aredestroyed. The assembly, which must be carried out very carefully,requires additional personnel and causes additional cists, particularlyfor objects whose cavities are small and difiicult of access.

The present invention clearly avoids the disadvantages of the prior artand is more fully illustrated in FIGS.

to 10. In FIG. 5, a can 2 to be coated is immersed into coatingvessel 1. The can 2is provided with anopening 4 in the top surface andan opening 5 in the bottom surfaceand is electrically connected by leadwire 3 to the positive pole of-adirect current source andthe vessel 1 isconnected to the negative pole thereof. An auxiliary bipolar electrodeconsisting of copper wire 7 which passes through the side wall of can 2at point 8 whichis insulated and outside the can the copper wire 6 isattached to an iron wire mesh 9. This auxiliary electrode is not'ingalvanic contact with the current source. The outer part of the bipolarauxiliary electrode 9 is arranged in the proximity of the cathode andacts therefore as 'an anode, while the part 7 in the interior of the can2 acts as a cathode with respect to the latter. The entire surface ofthe can 2 as well as the outer part 9 of the bipolar auxiliary electrodeare coated.

After the coating, the coated can 2 is lifted out of the bath, andrinsed with water. The auxiliary electrode is removed from the canwithout damaging the film layer. The quality 'of the stoved lacquer coatinside the can 2 is of the same quality as the layer on the outer wallof the can. The coated wire mesh 9 is discarded.

FIG. 6 shows the equivalent circuit diagram of this arrangement wherethe points 1a to 8a have the same meaning as the points 1a-8a in FIG. 2.10a denotes the resistance of the bipolar auxiliary electrode, which issubstantially lower than that of the liquid column 6a. As it can beseen, there is always a relatively high cathode potential at point 11a,in contrast to the arrangement according to FIGS. 2 and 4, which attainswith increasing outside coating practically the entire applied directcurrent voltage, without the bipolar auxiliary electrode being in anyway in metallic contact with the cathode.

In the embodiment of FIG. 7, a can 2v to be coated is immersed in acoating vessel 1 provided with an electrode 8' connected to the negativepole of a direct current source. Can 2 by lead wire 3 is connected tothe positive pole of the said current source and is provided with anopening 4 in the top surface and an opening 5 in the bottom surface. Anauxiliary bipolar electrode 5b made of copper passes through the wallsof can 2 at points 10 and 11 which are insulated. The inside and outsideof the can are both then coated.

FIG. 8 illustrates the invention when alternating current is used forthe coating process. The electrocoating bath vessel 1 is made ofaluminum with an interior coating of aluminum oxide and is connected tothe alternating current source. Vessel 1 in combination with the coatingbath acts as a rectifier. The hollow body or can 2 to be coated isconnected to the other pole of the alternating current source by leadwire 3 and is provided with opening 4 in the top surface and opening 5in the bottom surface. The said object 2 acts as an anode. The auxiliaryelectrodes are comprised of a plate-shaped outer part 12 made ofnoncoatable material which is connected to inner part 13 which passesthrough the wall of object 2 at point 14 which is insulated. The innerpart 13 of the auxiliary electrode acts as a cathode with respect toobject 2 and effects the complete coating of the interior.

In the embodiment of FIG. 9, two car bodies 12 and 13 are to be coatedand are suspended one behind the other from a conveyor rail 14 incoating vessel 1. The coating vessel 1 acts as a cathode and the carbodies are always in the bath at the same time and move through the bathin the direction of the arrows. Car body 13 is galvanically connected bycurrent collecting rail 15 to the current source and car body 12 is notin the direct circuit. The current source is an impulse generator whichgenerates current in the form of rectangular impulses with a maximumvoltage of 400 v., an impulse duration of 30 milliseconds and an impulseinterval of 30 milliseconds.

Car bodies 12 and 13 are provided with auxiliary electrodes 16 and 17,respectively, of copper which is conducted insulated through the wall ofthe car body at point 18. Between point 18 and roller mechanism 19, theauxiliary electrode is coated with an insulating mate rial. The bipolarauxiliary electrode 17 is in galvanic contact with car body 12 by meansof rail 20. In this position, the body 13 acts as an anode with respectto vessel 1. Body 12 represents the large-surfaced part of the bipolarauxiliary electrode 17. This part of the bipolar auxiliary electrodeacts therefore as an anode with respect to vessel 1, and the coating ofthe outer surface of body 12 which is not in the direct current flow,starts.

The part of the auxiliary electrode 17 inside body 13 acts as a cathodewith respect to body 13, so that the uniform coating of the interior isensured. As soon as the coating of body 13 is completed, body 12 entersthe direct circuit in its place. Auxiliary electrode 16 is in galvaniccontact with the following body. Since body 12 is already somewhatcoated, the current surge is not great when the current is turned on forthis body 12. In previously known methods, however, the uncoated bodyhad to be introduced under reduced voltage in order to avoid anexcessive current surge at the start of the coating operation.

FIG. 10 illustrates a method of coating car bodies with direct currentusing bipolar auxiliary electrodes and an auxiliary cathode called aframe cathode. The coating is effected by means of direct current. Body12 is introduced on the live rail 14 into the coating bath and acts asan anode. Vessel 1 acts as a cathode. Frame cathode 21 is inserted inbody 12, insulated against the latter. The frame cathode is connected tothe same pole of the DC source as the vessel and acts therefore as acathode. Furthermore, two bipolar auxiliary electrodes 22, 23 areprovided. The bipolar auxiliary electrode 22 serves to coat a smallcavity separated from the large cavity of the body by partition 24 andit is conducted insulated through partition 24. The large-surfaced partof the bipolar auxiliary electrode 25 is arranged close to the frameelectrode and acts as an anode with respect to the latter, the otherpart 26 protrudes into the small cavity of the body and acts as acathode with respect to the latter. The bipolar auxiliary electrode 23protrudes partly into the coating bath outside the body in the proximityof the main cathode, partly into the interior of the body. Thisarrangement of the electrodes and auxiliary electrodes effects a uniformcoating of the entire surface of the car body.

Various modifications of the method and apparatus of the invention maybe made without departing from the spirit or scope thereof. Forinstance, the method is also suitable for water-soluble orwater-dispersible coating agents which are deposited on the cathode withthe corresponding reversal of the polarity of the current sources.

We claim:

1. A method for electrocoating conductive surfaces of a hollow body witha coating material which comprises immersing a hollow object having atleast an electrically conductive surface which acts as an electrode inan electrically conductive coating bath containing an organic coatingagent to be deposited on the hollow object and provided with acounter-electrode, passing an electric current through the said coatingbath and removing the coated hollow object from the coating bath, thecoating of the interior of the hollow object being effected by at leastone bipolar auxiliary electrode with one pole inside the hollow objectand one pole outside the hollow object, a pole of which is coated duringthe process.

2. The method of claim 1 wherein at least one additional electrode isalso in the cavity of the hollow body and has the same potential as thecounter-electrode.

3. The method of claim 1 wherein the portion of the bi-polar electrodewhich is coated during the process has a large surface area.

4. The method of claim 1 wherein the hollow body acts as an anode andcounter-electrode is the cathode.

5. The method of claim 1 wherein at least a portion of the bipolarelectrode is made of a non-coatable material.

6. The method of claim 1 wherein interior coating thickness is regulatedby the ratio of the surface area of the cavity to the surface area ofthe coated part of the bipolar electrode.

7. The method of claim 1 wherein the bipolar electrode is in galvaniccontact with the hollow body to be coated when dipped into the bath butnot in galvanic contact with a current source.

8. An apparatus for the electrocoating of a hollow object having anelectrically conductive surface at least with a coating agent comprisinga hollow body adapted to act as an electrode and whose surface at leastis electrically conductive, an electrically conductive coating bathadapted to an organic coating agent which is to be deposited on thehollow body, a counter-electrode adapted to be immersed in the saidcoating bath, an electrical source connected to the hollow body and thecounter electrode and at least one bipolar auxiliary electrode adaptedto extend partially into the interior of the hollow body and partiallyoutside the hollow body and not in electrical contact with the hollowbody.

i 9; The "apparatusxof claim 8 wherein the"por tion of the bipolarelectrodes adapted to-extend outside the hollow body is many timesgreater in areatha'n the portion of the bipolar electrode adapted toextend into the hollow body. 10. The apparatus of-claim '8 wherein-thebipolar electrode is made of copper.

. 11.. The apparatus of claim 8wherein the bipolar elecw:

References Cited V 1 UN TED STATES PATENTS; 3,418,233

12/1968 Igvas et 51." 204 30d 3,4s3,097 12/1969 Bush et a1. 204 1s1x3,476,667

11/1969 Gilchrist 20418l HOWARD. S. WILLIAMS, Primary Examiner

